The present invention relates to an assembling method for an optical system of an optical pick-up in which a semiconductor laser is employed.
In general, an optical system of an optical pick-up is configured to converge a laser beam emitted by a semiconductor laser on an optical disc. Since in general the semiconductor laser causes an astigmatism, the laser beam obtained by converging the laser beam emitted by the semiconductor laser through the optical system has an astigmatism.
More specifically, a shape of a beam spot formed on the optical disc by the optical system changes from a circular shape state to an elliptical shape state depending on a defocus state of the laser beam on the optical disc, which raises a problem, such as deterioration of an RF signal or a jitter property. For this reason, the optical system is required to decrease the astigmatism.
Japanese Patent Provisional Publication No. 2004-342145A (hereafter, referred to as JP2004-342145A) discloses a technique for canceling an astigmatism caused in components in an optical system excepting a collimator lens with an astigmatism caused by the collimator lens. More specifically, the astigmatism caused in components in the optical system excepting the collimator lens is cancelled with the astigmatism caused by the collimator lens by adjusting the position of the collimator lens by rotating the collimator lens about an optical axis of the collimator lens. In this case, the collimator lens is required to be designed to have a certain astigmatism corresponding to the amount of the astigmatism that the optical system has. It is understood that since the amount of astigmatism varies from an optical system to another, a collimator lens designed for a certain optical system is not always suitable for other optical systems.
Recently, new technical standards, such as a BD (Blu-ray Disc) and a HD DVD, have been proposed. Based on the fact that the diameter of a beam spot is inversely proportional to a numerical aperture defined on an image side in an optical system (hereafter, frequently referred to as an image side numerical aperture) and is proportional to the wavelength of a beam, such a new technical standard is designed to decrease the beam spot diameter by increasing the image side numerical aperture of an optical system and decreasing the wavelength of a beam of a light source in the optical system.
On the other hand, the amount of astigmatism increases in proportion to the square of the image side numerical aperture. Therefore, if the image side numerical aperture of an objective lens in the optical pick-up is increased, the entire astigmatism caused in the optical system by a semiconductor laser or manufacturing errors increases.
In general, aberrations including an astigmatism caused in an optical system are evaluated in the unit of a wavelength. Considering that an evaluation wavelength for the new technical standard is approximately 405 nm which is shorter than that for the DVD (approximately 660 nm) and the CD (approximately 790 nm), the amount of aberration in the optical system according to the new technical standard increases in inverse proportion to the used wavelength even if an error in the shape of the objective lens in the optical system according to the new technical standard is substantially the same as that for the optical system for the DVD or CD.
By configuring an objective lens having a small amount of astigmatism, it may be possible to suppress the entire astigmatism in an optical system. However, it is considerably difficult to manufacture an objective lens of which astigmatism is very small even if the designed shape of the objective lens exhibits a small amount of astigmatism. In general, it is difficult to avoid an astigmatism due to a manufacturing error. In addition, the amount of astigmatism due to a manufacturing error varies from an objective lens to an objective lens.
In general, a maximum admissible astigmatism is determined for manufacturing of objective lenses. If the number of manufactured objective lenses not satisfying the maximum admissible astigmatism is relatively large (i.e., the yield is relatively low), a manufacturer is required to decrease the manufacturing error of each objective lens. However, to decrease the manufacturing error, the manufacturer needs to improve a production line, which also increases the cost for manufacturing objective lenses. For this reason, it is desirable for the manufacturer that the maximum admissible astigmatism is as large as possible so as to prevent the deterioration of the yield and the increase of the manufacturing cost.